(1) Field of the Invention
This invention is in the field of char fuel gas producers wherein a char fuel, such as coal, is reacted with air, or other oxygen containing reactant gas, to form producer gas as product.
(2) Description of the Prior Art
General descriptions of prior art char fuel gas producers are presented in the following reference:
This reference is incorporated herein by reference thereto.
In a char fuel gas producer, char fuel at high temperature (circa 1500.degree. F. to 2000.degree. F.) is reacted with oxygen from air or other source to form producer gas, one of the principal fuel constituents of which is carbon monoxide. When steam is also used as an additional reactant gas the producer gas will additionally contain hydrogen as a fuel constituent. Pure oxygen or oxygen enriched air are sometimes used as reactant gas to avoid or minimize the presence of diluent nitrogen gas present in the producer gas product. The resulting producer gas product is thus a fuel gas and can be used to fuel industrial furnaces or boilers or engines or electric power plants.
As used herein and in the claims the term char fuel is as defined in U.S. Pat. No. 4,509,957, col. 2, line 58 through 68, and in U.S. Pat. No. 4,455,837, col. 4, line 8 through line 16, and this material is incorporated herein by reference.
A char fuel gas producer comprises a container within which is a char fuel reaction chamber containing char fuel. Oxygen containing reactant gas is supplied into the char fuel reaction chamber to react in a primary reaction with the hot char fuel therein. The resulting producer gas product is removed from the char fuel reaction chamber.
A means for preheating the char fuel within the char fuel reaction chamber is used to bring the char fuel up to that rapid reaction temperature at which it will react rapidly with oxygen in, those gases adjacent to the char fuel surface; while the plant is being started. Thereafter the means for preheating the char fuel can be turned off when the heat of the primary reaction becomes sufficient to keep the char fuel at or above this rapid reaction temperature.
The term "rapid reaction temperature" is used herein and in the claims to mean that temperature of the char fuel at which it will react with the supplied reactant gas containing oxygen gas sufficiently rapidly to maintain the char fuel temperature at or above this rapid reaction temperature due only to the heat of the reaction between the char fuel and this reactant gas. This rapid reaction temperature varies with the kind of char fuel being reacted, the oxygen content of the reactant gas, and the operating conditions prevailing within the char fuel reaction chamber.
For the same reactant gas and operating conditions different char fuels have different rapid reaction temperatures, some charcoals reacting rapidly with air in usual type reactors at temperatures as low as 1200.degree. F. whereas some petroleum coke fuels will only react rapidly with air at temperatures above about 1500.degree. F.
For a particular char fuel and operating condition a higher rapid reaction temperature is required when the oxygen content of the reactant gas is reduced since more of the heat of char and oxygen reaction is diverted to the heating tap of non reactive portions of the reactant gas. Below a certain minimum oxygen content the reaction between the char fuel and the oxygen is too slow to sustain itself by its own heat of reaction, and the term "appreciable oxygen gas content" of reactant is used herein and in the claims to mean an oxygen content greater than this minimum value. Ordinary air, with an oxygen gas content of about 21 volume percent, will usually react readily with most commonly available hot char fuels in reasonably well insulated reaction chambers, and is an example of a reactant gas containing appreciable oxygen gas suitable for use in most gas producers. In some gas producer applications oxygen enriched air or essentially pure oxygen has been used as the reactant gas containing appreciable oxygen gas. Reactant gases containing less oxygen than air, while theoretically useable in gas producers, have rarely, if ever, been so used.
As the char fuel reaction chamber becomes smaller, external heat loss rate increases, and the char fuel must be brought to a higher temperature, and thus higher reaction speed, in order for the char fuel and oxygen gas reaction to be self sustaining. We thus see that the rapid reaction temperature is not a property of the char fuel alone and can only be determined experimentally within the reaction chamber to be used, and with the oxygen containing reactant gas to be used.
As char fuel is reacted to ashes within the char fuel reaction chamber it is replaced by a refuel mechanism means for supplying fresh char fuel into a refuel end of the chamber. The char fuel is thus moved along through the char fuel reaction chamber toward an opposite ash collection end of the chamber. Hence the char fuel being reacted within the char fuel reaction chamber has a direction of motion from the refuel end toward the ash collection end. An ash removal mechanism is used as a means for removing ashes from the char fuel reaction chamber.
As the char fuel within the char fuel reaction chamber moves along in the char fuel motion direction, it is preheated by heat transfer from char fuel portions which are further along and are reacting rapidly with oxygen and thus are at high temperature. Where the char fuel being used is essentially free of volatile matter, as with coke fuel, this preheat zone serves to bring the new char fuel up to its rapid reaction temperature. The char fuel then enters the rapid reaction zone, and carbon reacts therein with oxygen to form producer gas. Beyond the rapid reaction zone in the direction of char fuel motion the char fuel is essentially completely reacted to ashes which pass into an ash collection zone at the end of the char fuel motion path.
When the char fuel being used contains volatile matter, as with bituminous coal, the char fuel preheat zone also serves as a volatile matter distillation zone to remove the volatile matter from the coal, in part by distillation and in part by reaction to volatile products. In the absence of oxygen, appreciable portions of this distilled volatile matter become tars and other portions become fuel gases of essentially hydrocarbon or oxygen containing hydrocarbon type. These tars from coal volatile matter are undesirable in char fuel gas producers as they tend to clog piping and apparatus and are difficult to burn fully in the subsequent uses of the producer gas product.
In prior art gas producers tar formation from coal volatile matter has been successfully reduced by passing the reactant air first into the preheat and volatile matter distillation zone. The emerging volatile matter apparently reacts with oxygen in the air to form oxygenated hydrocarbon type materials which form much less tar. The resulting volatile matter-in-air mixture then passes into the rapid reaction zone. Within the rapid reaction zone the volatile matter-in-air mixture is apparently burned in appreciable part to fully reacted carbon dioxide and steam. The carbon dioxide and steam, plus any unreacted oxygen, then react with carbon in the rapid reaction zone to form producer gas which emerges from the char fuel reaction chamber. One disadvantage of this method for reducing tar formation is that the initial burning of the volatile matter in air mixture on entering the rapid reaction zone creates very high temperatures there and ash fusion and clinkering may result. These clinkers clog up the motion of the char fuel along the char fuel motion direction and may encase carbon particles and thus prevent complete carbon gasification. Another disadvantage of this method for reducing tar formation is that the carbon dioxide and steam created by burnup of the volatile matter-in-air mixture, react much more slowly with hot carbon in the rapid reaction zone to form producer gas.
The volatile matter in air mixture is a gaseous fuel which has a higher energy content per unit volume than that of a producer gas in air mixture. It is a disadvantage of prior art char fuel gas producers that this valuable volatile matter in air mixture is consumed within the gas producer.